Spots and spin The Sun is now at solar max, but you can rest easy -- the Earth should be safe from being fried by a superflare, a new study suggests.

Research reported on the pre-press website ArXiv.org found these massive eruptions, a million times more powerful than solar flares, usually occur on fast-spinning stars that have large sunspots.

Lead author Yuta Notsu of Kyoto University and colleagues, analysed data from NASA's Kepler Space Telescope to detect changes in brightness of Sun-like stars known to produce super flares.

"Superflares whose energy is 100 to 1000 times larger than that of the most energetic flare on the Sun, can occur on solar-type stars once in a few thousands of years," they write.

Solar flares are the most energetic explosions on the surface of the Sun.

They form when magnetic field lines twisted by the Sun's rotation break through the surface, causing cooler local temperatures which appear as dark spots.

As these magnetic field lines snap, they fling out electro-magnetic radiation, protons, electrons and heavier atomic nuclei.

Flares are also known to occur on other types of stars, such as young stars or binary stars which have been known to produce superflares.

These stars spin very fast.

Until recently it was thought that superflares did not occur on slow-moving stars such as our Sun, but in 2000, astronomers detected superflares from nine Sun-like stars.

To explore how often and why superflares occur in Sun-like stars, Notsu and colleagues examined 90,000 of these stars observed by Kepler, which can detect very subtle changes in stellar brightness.

They identified 365 superflare events on 148 stars that have surface temperature of 5100 Kelvin.

They then analysed the brightness variation data for details about sunspot activity and stellar rotation.

"Our results indicate that these brightness variations...can be explained by the rotation of the star with fairly large starspots," they report.

They found the energy released by the superflare was related to the size of the starspot.

They also found that although stars with slow rotation rates can still produce flares that are as energetic as fast-spinning stars, the average flare frequency was lower.

"These results suggest that the energy of superflares can be explained by the magnetic energy stored around starspots."

Unanswered issues

Our Sun rotates slowly and produces weak magnetic fields.

But there are still questions about how big a solar flare from our Sun can get, says the Ionospheric Prediction Services space weather science officer Dr Matthew Francis.

"We still don't have as good an understanding of how solar flares work as we would like," says Francis.

"One of the things we don't know about our Sun from a space weather perspective is: how bad can it get."

And that's important for understanding the level of impact space weather can have on navigation and communications systems, power grids and orbiting spacecraft, says Francis.

"When we're engineering satellites or systems on the ground to be able to withstand the worst possible space weather events, we don't know what that upper limit is," he says.

Space weather events are gauged by sunspot activity and are closely linked to the Sun's 11-year solar cycle which climaxes this year, which is now at its peak.

"This has been a fairly mild solar maximum," says Francis.

"The two hemispheres of the Sun are behaving quite differently in this solar cycle...there's a double peak happening, one hemisphere has clearly already peaked and is now declining, and the other's about to do that."